Colour variations in the GRB 120327A afterglow

A. Melandri , S. Covino , E. Zaninoni , S. Campana , J. Bolmer , B.E. Cobb , J. Gorosabel , J.-W. Kim , P. Kuin , D. Kuroda , D. Malesani , C.G. Mundell , F. Nappo , B. Sbarufatti , R.J. Smith , I.A. Steele , M. Topinka , A.S. Trotter , F.J. Virgili , M.G. Bernardini (1) , P. d'Avanzo , V. d'Elia , D. Fugazza , G. Ghirlanda , A. Gomboc , J. Greiner , C. Guidorzi , J.B. Haislip , H. Hanayama , L. Hanlon , M. Im , K.M. Ivarsen , J. Japelj , M. Jelínek , N. Kawai , S. Kobayashi , D. Kopac , A.P. Lacluyzé , A. Martin-Carrillo , D. Murphy , D.E. Reichart , R. Salvaterra , O.S. Salafia , G. Tagliaferri , S.D. Vergani (2)

1 LUPM - Laboratoire Univers et Particules de Montpellier
2 GEPI - Galaxies, Etoiles, Physique, Instrumentation

Aims. We present a comprehensive temporal and spectral analysis of the long Swift GRB 120327A afterglow data to investigate possible causes of the observed early-time colour variations.Methods. We collected data from various instruments and telescopes in X-ray, ultraviolet, optical, and near-infrared bands, and determined the shapes of the afterglow early-time light curves. We studied the overall temporal behaviour and the spectral energy distributions from early to late times.Results. The ultraviolet, optical, and near-infrared light curves can be modelled with a single power-law component between 200 and 2 × 104 s after the burst event. The X-ray light curve shows a canonical steep-shallow-steep behaviour that is typical of long gamma-ray bursts. At early times a colour variation is observed in the ultraviolet/optical bands, while at very late times a hint of a re-brightening is visible. The observed early-time colour change can be explained as a variation in the intrinsic optical spectral index, rather than an evolution of the optical extinction.Key words: gamma-ray burst: individual: GRB 120327A / dust, extinction⋆ Table 2 is only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/607/A29⋆⋆ Deceased.

Mots clés: gamma-ray burst: individual: GRB 120327A; dust; extinction

Domaines: Astrophysique [astro-ph]

A. Melandri, S. Covino, E. Zaninoni, S. Campana, J. Bolmer, et al.. Colour variations in the GRB 120327A afterglow. Astronomy and Astrophysics - A&A, 2017, 607, pp.A29. ⟨10.1051/0004-6361/201731759⟩. ⟨hal-01645283⟩